JSFiddle - React, Tailwind, and code Playground

HTML

<form name="reg_form" method="post" action="ins_data.php">
          <table width="530" border="1">
              <tr>
                <td class="text"><p>Логин:</p></td>
                <td class="inputs"><input type="text" id="login_field" name="login_field"/></td>
                <td><div class="message" id="login_message"></div></td>
              </tr>
           </table>
</form>

JavaScript

var login_field = document.getElementById('login_field');
login_field.oninput = function()
        {
            document.getElementById('login_message').innerHTML = "Проверка!!!";
        }